The purpose of the proposed research is to provide data that will aid in understanding how behavior is adjusted to changing environmental stimulation. These data will be used to test and extend a theory developed to account for processes through which organisms learn to recognize, and respond appropriately to, environmental stimuli that are differentially associated with behaviors that lead to reward. Attention will be focused upon situations in which gradations in stimulation are correlated with gradations in the required behavior. The experiments are concerned with absolute identification by monkeys of a series of stimuli that form a continuum (here squares varying in size and lights varying in luminance). In the size identification experiments the stimuli will be displayed on a computer monitor and a joystick will be used to move the cursor to a marked or unmarked position, on the display, that has been defined as correct for the particular stimulus to be identified. The sequence of behaviors preceding the decision will be continuously monitored by observing moment- to-moment changes in force exerted by the monkey on the stick. In the experiment using unmarked positions, the monkeys will have the option of obtaining additional visual information prior to their final choice. This technique will make possible an analysis of some previously "invisible" aspects of the decision process. The luminance identification experiments are designed specifically to provide comparative data on limitations in accuracy of absolute identification. It is expected that these data will help define similarities and differences in the way humans and other animals use information regarding past events.